Typical engine control systems for automobiles generate a spark control signal which is used to initiate the charging and discharging of an ignition coil. At the proper time, the high voltage generated by the ignition coil causes a spark plug to fire and ignite the fuel in its associated cylinder.
The same engine control system may also include a diagnostic section that senses the initial charging of the ignition coil to confirm that a spark control signal was generated, and that the ignition coil was responding by beginning to charge.
While such diagnostics are useful, they do not sense other types of ignition malfunctions. Specifically, they do not detect the type of spark plug misfire which can occur even though the ignition coil has been properly charged.
Spark plug misfires can occur due to a faulty spark plug, an open plug wire, a short circuit in the ignition system, or the like. When the misfire happens, unburnt fuel travels from the engine to the catalytic converter, causing the temperature of the catalyst to rapidly increase to an undesirably high level. In addition, the unburnt fuel removes oil from the sides of the piston associated with the misfiring spark plug, thus promoting increased wear. Poorer mileage also results.
Accordingly, it is a general object of the invention to provide an improved method and apparatus for detecting spark plug misfire so as to overcome the shortcomings discussed above.
It is a more specific object of the present invention to provide a technique and a circuit for reliably sensing spark plug misfire due to open circuits and/or short circuits in the secondary of an automotive ignition system, and for optionally shutting down the flow of fuel to the cylinder that is misfiring.